TVLine confirmed that fellow Glee alum Criss will play the Neil Patrick Harris-originated Music Meister in the two-part March 20-21 crossover, having previously nursed a crush on Gustin’s sinister Sebastian back in their Glee days. Executive producer Andrew Kreisberg was understandably enthused by the three-way connection:

With our Flash and our Supergirl being Glee alums, how could we not have them go up against another Glee favorite like Darren Criss? We have been blown away by his talent over the years and we can’t wait to see what he brings to the Music Meister.

Dermott Downs will direct the episode, while executive producer Berlanti said earlier in 2016 they’d only just begun the process of clearing music and drafting original songs, and approaching “someone we’re really excited about.”

There’s plenty more Supergirl and Flash in the meantime, so stay tuned for further details of the upcoming “Duet.”